' renders a string as plain text, whereas " tells php to render variables inside the string meaning it takes longer to process plain text like you were doing.

From an HTML validation perspective, no. This is absolutely not a *requirement.* You can use single or double quotes and it is still perfectly valid HTML either way. The convention is to use double quotes, but it's still a matter of taste until W3C creates a standard that says otherwise and even then, it will only be mandatory in that particular standard.